1. Field of invention
The invention concerns a device for production of baqs, and a method for the production of bags that employs the device, as described herein.
2. Description of the Prior Art Devices and methods of the type just mentioned have been known for many years. German Patent DE 720 665 discloses a machine for production of flat or gusseted bags, in which a material web is unwound from a reel, made into a tube and separated into individual tube pieces, each of which is then provided with a bottom.
In a variant shown there a web is initially provided with transverse perforations, in which at each perforation individual perforation sections are offset relative to the other sections in the running direction of the web. After creation of the tube and tear-off of individual tube sections this offset perforation means that parts of the rear wall on one end of the tube section protrude beyond the front wall. This part of the rear wall protruding beyond the front wall is also referred to as a tab. The front wall protrudes beyond the rear wall on the other end accordingly. The perforation is generally configured so that the rear wall protrudes beyond the front wall on the leading end of the tube section. This process is chosen because the tab can then be simply folded back and fastened to the front wall, for example, glued. Parts of the front wall are also often folded together with the tab and glued, which increases the tightness of the bag and the durability of the bottom. The rear wall for this purpose need only run on a nonmoving resistance. The bags produced this way are referred to subsequently as “ordinary bags.”
However, in the recent past bags in which a strip of transparent material is introduced in the front wall in the longitudinal direction have increasingly been required. The rear wall folded onto the front wall is generally printed, since the printable surface on the front wall is smaller overall in comparison with ordinary bags. However, it is often also desired in this type of bag that the rear wall also protrudes beyond the front wall on the rear open end in order to be able to easily fill such a bag.
Ordinary bags are simply rotated around the longitudinal axis for this purpose so that the rear wall becomes the front wall and vice-versa. However, the tab with which the bag is closed is then arranged on the (new) rear wall. However, printable area is then lost in bags with a transparent strip.
In order to create a bag as desired, it is therefore required to remove a material section on the open end of the front wall so that the rear wall here also protrudes beyond the front wall.
Machines are already known from practice with which this material section could be removed from the front wall from the trailing end. This occurs together with the work step of separation of a tube section from the tube. The leading end of the tube is grasped by the roll gap of a roll pair, which is often referred to as tear-off roll pair. Since the rolls of this roll pair have higher peripheral speed in the rolls of the last advance roll pair, the tube section tears off along the perforation line. At the same time the part of the front wall protruding beyond the rear wall is grasped and held so that this part tears off from the tube section being separated and also from the new leading end of the tube. An additional roll pair is used to hold this part, the peripheral speed of the rolls being lower than the peripheral speed of the rolls of the tear-off roll pair but greater than the peripheral speed of the rolls of the advance roll pair. These additional rolls can be viewed as an additional tear-off device. Such devices are known from document DE 647 889 B.
The described device, for which no document is known to the applicant, operates very slowly in comparison with devices that produce ordinary bags. The separated part also cannot be reliably removed frequently so that disturbances in the machine up to shutdown can occur. Adjustment of the three mentioned roll pairs relative to each other is very difficult to configure. In addition, the required speed differences often pose problems and restrict possible section lengths.